1. Field of the Invention
The present invention provides a differential wavelength imaging method for chemical analysis and infrared imaging, and more particularly, to detect concealed explosives, drugs, or other contraband materials, hidden under clothing, at security checkpoints at airports, border crossings, government buildings, and military facilities, and identify same. Specifically, the present invention provides a method of differential wavelength imaging capable of standoff detection and identification of contraband materials when they are concealed on a person under clothing, in a backpack, or concealed in bags not worn on a person, and identify same using NIR wavelength differential imaging techniques, and system for carrying out same.
2. Description of the Related Art
Numerous conventional approaches have been taken in the field of standoff detection and identification to attempt to detect and identify materials, especially explosives, drugs, etc., concealed under clothing. Such conventional approaches that have been reported in the literature for standoff detection and identification of concealed contraband materials include: x-ray backscatter imaging, neutron excited gamma ray emission spectroscopy, terahertz reflection spectroscopy, and laser induced breakdown spectroscopy.
Literature references concerning these approaches include: National Research Council of the National Academies, “Existing and Potential Standoff Explosives Detection Techniques,” National Academies Press, Washington D.C., (2004); C. Bruschini, “Commercial Systems for the Direct Detection of Explosives (for Explosive Ordnance Disposal Tasks,” ExploStudy, Final Report, Swiss Defense Procurement Agency, http://diwww.epfl.ch/lami/detec/explostudy.html, (2001); “Terahertz waves penetrate the world of imaging,” Opto & Laser Europe, October (2002), http://optics.org/articles/ole/7/9/5/1; Martens and T. Naes, Multivariate Calibration, J. Wiley & Sons, New York (1989); H. Seisler, Y. Ozaki, S. Kawata, and H. Heise, Near-Infrared Spectroscopy—Principles, Instruments, Applications, chapter 7 “Chemometrics in Near-Infrared Spectroscopy,” Wiley-VCH, (2002); H. Mark, et. al., Analytical Chemistry, 57, 1449 (1985); C. Lopez-Moreno, S. Palanco, J. Javier Laserna, F. DeLucia Jr, A. W. Miziolek, J. Rose, R. A. Walters and A. I. Whitehouse, “Test of a stand-off laser-induced breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces,” J. Analytical Atomic Spectrometry, 21, 55-60 (2006).
Problems with the x-ray backscattering imaging approach include: poor chemical selectivity for chemical identification with high potential for false positives, large size and weight of instrumentation which prevents the system from being man-portable, and human health risk from x-ray exposure.
Problems with neutron excited gamma ray spectroscopy include: limited chemical selectivity resulting from the measurement only producing elemental concentration results, limited sensitivity, and long measurement times at significant standoff distances (i.e. 1 ft. or greater), and substantial human health risks. Measurements providing only elemental analysis information would not be likely to be able to identify explosive materials such as triacetaone-triperoxide that contain only the elements C, H, and O, and identification of drugs would be very difficult.
Problems with terahertz spectroscopy include: slow measurement time, as well as substantial problems with interference from absorption of terahertz radiation by atmospheric water vapor for standoff distances greater than 10 ft. In addition, the size and weight of the equipment are too great for man-portability.
Laser induced breakdown spectroscopy (LIBS) is a trace detection method that can detect and identify small particles of explosive or other materials on the outside of a surface in a standoff mode. The primary problem with LIBS is that it cannot detect or identify materials concealed underneath a covering layer such as cloth and can only detect explosive particles on the outside surface of clothing. Explosives or other contraband materials that are well sealed in a plastic bag and concealed under clothing, where the outside surface of the clothing was not contaminated with the dust of the contraband material, could not be detected or identified with LIBS.
Further, NIR spectroscopy has been used to identify chemical compounds. In particular, Li, et al., in U.S. Patent Publication No. 2005/0010374, discloses a method of analyzing NIR data, so as to identify various solid forms of chemical compounds and drug candidates. This method includes the steps of: (1) computing the second derivative spectra for collected NIR spectra; (2) applying principal component analysis (PCA) of the second derivative spectra at predetermined wavelengths either the entire wavelength region or a selected wavelength region for segregating the samples; identifying the groups and group membership from the PCA graph, and further evaluating group members by calculating Mahalanobis distances of a given group to assess qualification of the group members. However, this method is merely an initial exploratory analysis of near-infrared spectra designed to identify how many different components or materials are present in an unknown sample, and how different their spectra are.
Additional conventional methods include using NIR spectroscopy to attempt to identify components relative to a saved calibration library, via identification of absorption wavelengths, and comparison thereof to known standards. For example, U.S. Pat. No. 7,239,974, to Gulati, discloses an explosive device detection method and system based on differential emissivity. This method and system monitors the emissivity levels of target subjects in monitored zones by repeatedly scanning the pixels of an infrared photodetector array, and then processing the pixel values to determine if they correspond to at least one calibrated emissivity level associated with a concealed explosive device. The calibration techniques of Gulati involve attempts to eliminate the effects of clothing and other personal items, as well as environmental factors, but suffer from a concentration mainly on differences in emissivity levels caused by distance of the target from the source (IR photodetector), rather than increasing the contrast/difference in measured emissivity between the covering materials and the concealed contraband materials.
Further, such conventional methods are inaccurate, when used to attempt to identify materials concealed under clothing, covering materials, etc., due to the difficulties inherent in filtering out the wavelengths reflected from the clothing, covering materials, containment materials, etc., as well as, importantly, ambient light, sunlight, etc. Thus, to obtain accurate measurements, such conventional NIR methods generally are confined to laboratory or laboratory-like environments, not public areas, such as airports.
In view of the above, it is an object of the present invention to overcome the disadvantages of the prior art, as described above, and provide a method to efficiently and accurately detect and identify concealed materials, such as explosives, drugs, or hazardous materials, concealed on a person under clothing or in a backpack, or concealed in unattended paper, plastic, cloth or leather bags (including backpacks), and a system for carrying out same. The detection method of the present invention is preferably performed at some finite distance from the material being detected, which is referred to as the “standoff distance”. The standoff distance could be in the range of from 1 cm to 100 m. In all cases, the material being detected may be concealed under some type of covering materials such as cloth, paper, plastic, or leather that has substantial optical absorption and/or light scattering properties which obscures viewing the concealed material under the covering material with light in the visible wavelength range (400-700 nm).
It is a further object of the present invention to provide such a system and method as described above, wherein the system and method may identify the concealed material without the use of reference images (such as dark images).